The purpose of this study was to evaluate nasal airway resistance (NAR) during rapid maxillary expansion (RME) using acoustic rhinometry (AR). The sample comprised 22 children (13 girls and nine boys) with maxillary constriction. The mean age was 12.9 +/- 1.54 years and all patients were found to have normal nasal cavities following anterior rhinoscopic examination. A modified bonded splint type RME appliance was used for expansion. AR was used to measure NAR before (T1), during (T2) and after (T3) expansion, and at the end of retention (T4). Each AR recording was performed, for each patient, with and without the use of a decongestant. Subjective evaluation of reported changes in nasal breathing were also undertaken at T3. The results showed that NAR was significantly reduced with the use of RME, with the main decrease observed during expansion (P < 0.05). The use of a decongestant was not found to have any effect on the results. Subjective evaluation showed that 59 per cent of patients considered that their nasal breathing had improved following RME.